High-performance carbon fibers can be classified into PAN-based carbon fibers obtained from polyacrylonitrile (PAN) and pitch-based carbon fibers obtained from pitches. Carbon fibers are widely used in aviation and space, construction and civil engineering, and sports and leisure applications, making use of their feature that they have much higher strength and elastic modulus than ordinary synthetic polymers.
The carbon fibers have a higher thermal conductivity than ordinary synthetic polymers and therefore are excellent in radiation performance. The carbon fibers attain a high thermal conductivity due to the movement of a phonon. The phonon conducts heat well in a material in which a crystal lattice is formed. It cannot be said that a crystal lattice is fully formed in commercially available PAN-based carbon fibers and their thermal conductivities are generally lower than 200 W/(m·K). It is hardly said that this is preferred from the viewpoint of thermal management. In contrast to this, a crystal lattice is fully formed in the pitch-based carbon fibers due to high graphitization and the pitch-based carbon fibers easily attain a higher thermal conductivity than the PAN-based carbon fibers.
As heat generating electronic parts are becoming higher in density and electronic equipment such as portable personal computers are becoming smaller, thinner and lighter, the requirement for the reduction of the heat resistance of radiating members used in these equipment is becoming higher and higher, and the further improvement of radiation properties is desired. The radiating members include heat conductive sheets composed of a cured product charged with a heat conductive filler, heat conductive spacers composed of a cured product having flexibility and prepared by charging a heat conductive filler into a gel-like substance, heat conductive paste having fluidity and prepared by charging a heat conductive filler into a liquid matrix, heat conductive paste having improved fluidity and prepared by diluting a heat conductive paste with a solvent, heat conductive adhesives prepared by charging a heat conductive filler into a curable substance, and phase change type radiating members making use of the phase change of a resin.
To improve the thermal conductivities of these radiating members, a heat conductive material should be charged into a matrix in a high concentration. Known heat conductive materials include metal oxides, metal nitrides, metal carbides and metal hydroxides such as aluminum oxide, boron nitride, aluminum nitride, magnesium oxide, zinc oxide, silicon carbide, quartz and aluminum hydroxide (Patent Document 1). However, metal-based heat conductive materials have high specific gravity and increase the weight of a radiating member. When a powdery heat conductive material is used, a network is hardly formed, thereby making it difficult to obtain a high thermal conductivity. Therefore, to improve thermal conductivity, a large amount of a heat conductive material must be used with the result that the weight and cost of a radiating member increase and it is hardly said that a heat conductive material is always convenient.
Therefore, to make effective use of the high thermal conductivity of a heat conductive material, it is preferred that the heat conductive material should form a network while a suitable matrix is existent therein. As for the shape of a heat conductive material for forming a network easily, a fibrous material is widely known (Patent Document 2).
An example of the fibrous material is a carbon fiber. The carbon fiber is used in carbon fiber reinforced plastics due to its stiffness and heat resistance (Patent Document 3). Also the use of the carbon fiber in secondary cell electrodes is proposed (Patent Document 4).
It is also proposed to use the carbon fiber in a heat conductive material. For example, Patent Document 5 proposes a radiating sheet comprising graphitic carbon fibers having an average fiber length of not less than 30 μm and less than 300 μm. Patent Document 6 proposes a heat conducting apparatus made of a composition comprising carbon fibers having a length of 10 to 150 μm. Patent Document 7 proposes a semiconductor device containing graphitic carbon fibers covered with a ferromagnetic material. However, Patent Documents 5 to 7 do not take into consideration the improvement of the dispensability of the carbon fibers in a matrix and there is room to improve the network forming capability of the carbon fibers to improve thermal conductivity.    (Patent Document 1) JP-A 2005-72220    (Patent Document 2) JP-A 2002-535469    (Patent Document 3) JP-A 7-90725    (Patent Document 4) JP-A 7-85862    (Patent Document 5) JP-A 2000-192337    (Patent Document 6) JP-A 11-279406    (Patent Document 7) JP-A 2002-146672